More Phone-home Uses
Other than phoning to the home (website) of the applications' authors, applications can allow their documents to do the same thing, thus allowing the documents' authors to trigger (essentially anonymous) tracking by setting up a connection which is intended to be logged. Such behavior, for example, caused v7.0.5 of Adobe Reader to add an interactive notification whenever a PDF file tries phoning (to its author's) home.
HTML e-mail messages can easily implement a form of "phoning home". Images and other files required by the e-mail body may generate extra requests to a remote web server before they can be viewed. The IP address of the user's own computer is sent to the web server (an unavoidable process if a reply is required), and further details embedded in request URLs can further identify the user by e-mail address, marketing campaign etc. Such extra page resources have been referred to as "web bugs" and they can also be used to track off-line viewing and other uses of ordinary web pages. So as to prevent the activation of these requests, many e-mail clients do not load images or other web resources when HTML e-mails are first viewed, giving users the option to load the images only if the e-mail is from a trusted source.
Read more about this topic: Phoning Home